Flywheels are generally known in the art for storing energy. While flywheel energy storage devices have been used for many years in satellite or other spacecraft applications, more recently they have been adapted for use on terrestrial machines. More specifically, hybrid power plants have been proposed which use a combustion engine as the primary mover and a flywheel as a secondary mover.
In some applications, the flywheel is operably coupled directly to an engine output, such as a crankshaft, upstream of a transmission. The flywheel may add to or subtract from power supplied by the engine to the transmission and, ultimately, one or more driven wheels. In this arrangement, the flywheel may also be configured to use regenerative braking, in which the flywheel is sped up to capture kinetic energy of the machine as it decelerates. Conversely, when the machine is accelerating, the flywheel may provide additional power to the wheels, thereby reducing flywheel speed. The position of the flywheel upstream of the transmission may allow it to efficiently spin up to a desired operating speed during start up. During regenerative braking, however, the energy from the ground engaging members may be transmitted through several mechanical connections, including the transmission, before it reaches the flywheel. Consequently, the amount of energy that can be captured by an upstream-located flywheel may be reduced by the mechanical losses as it travels through the transmission and other mechanical connections, thereby decreasing the efficiency of the flywheel.
In other applications, the flywheel may be operably coupled to a drivetrain output downstream of the transmission. When the machine decelerates, energy from the drivetrain (and an associated transmission) may be transferred to the flywheel. During acceleration of the machine, energy from the flywheel may be transferred to the powertrain to assist with the increased power demand. When positioned downstream of the transmission, the flywheel may more efficiently capture energy during regenerative braking due to the decrease in mechanical connections between the flywheel and the ground engaging members, thereby reducing the mechanical loss. During start up, however, the drivetrain may not be configured to spin up the flywheel during machine start up. Additionally, the location of the flywheel downstream of the transmission may make such spin up inefficient due to mechanical loss through the transmission.
U.S. Pat. No. 4,499,965 to Oetting et al. discloses a hybrid drive that includes both an engine flywheel and a storage flywheel. The engine flywheel is configured to compensate for non-uniformities in engine output torque, while the storage flywheel is configured to store energy during regenerative braking. Both the engine flywheel and the storage flywheel are positioned upstream of the transmission, and therefore suffer from the drawbacks noted above. Conversely, U.S. Patent Application Publication No. 2010/0152984 to Bowman et al. discloses a hybrid vehicle having a flywheel connected to a lower powertrain assembly, downstream of a transmission. The flywheel of Bowman et al. is not coupled to the engine, and therefore is incapable of engine-powered spin up.
The efficiency of a flywheel assembly further may be impacted by parasitic loads that resist rotation of the flywheel. Friction forces, for example, which resist the rotation of the flywheel body, are generally related to flywheel speed and environment. To reduce the impact of environmental factors, the flywheel body may be contained in a housing that is maintained at a partial vacuum pressure (i.e., substantially below atmospheric pressure) and at a reduced temperature. As a result, flywheel speed is often the primary contributing factor, and is generally proportionally related to, the amount of friction generated by the flywheel.
Friction losses may be exacerbated in more recent, high speed flywheel constructions. Flywheel bodies were conventionally formed of metal materials, such as iron and steel, which have a relatively high density that limits the speed at which such flywheels may be safely rotated before becoming structurally unstable. More recently, specialized flywheel materials have been developed to improve flywheel capacity. For example, flywheel bodies may be formed of carbon fiber material having a strength-to-weight ratio that is higher than the conventional metal materials. This permits rotation at higher speeds, such as up to approximately 60,000 rpm or more, thereby increasing energy storage capacity. The higher rotational speeds, however, may increase the amount of friction force and therefore decrease the efficiency of the flywheel assembly.
Some flywheel assemblies are known that use multiple flywheels to increase energy storage capacity. For example, U.S. Pat. No. 4,606,193 to Molina discloses a “Freewheel Flywheel Transmission System” having multiple freewheeling shafts coupled to an engine. An associated set of flywheels is directly connected to each freewheeling shaft, so that the flywheels in a given set rotate at the same speed. When a freewheeling shaft reaches a desired rotational speed, the next freewheeling shaft is engaged and brought up to speed. Once all of the freewheeling shafts are rotating at the desired speed, they are coupled to an alternator that is connected to an electric motor and a battery. While Molina discloses a system that uses multiple flywheels, it fails to consider parasitic loads, and resulting inefficiencies, associated with such a system.